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 Table of Contents  
Year : 2022  |  Volume : 11  |  Issue : 4  |  Page : 310-313

Diagnostic yield of direct smear, concentrated smear, liquid culture, and GeneXpert by sputum induction in smear-negative pulmonary tuberculosis patients

1 Department of Chest Medicine, Government Mohan Kumaramangalam Medical College, Salem, Tamil Nadu, India
2 Department of Chest Medicine, Saveetha Medical College, Chennai, Tamil Nadu, India
3 Department of Chest Medicine, Kanyakumari Government Medical College, Nagercoil, Tamil Nadu, India

Date of Submission16-Feb-2022
Date of Decision25-Apr-2022
Date of Acceptance27-Apr-2022
Date of Web Publication12-Nov-2022

Correspondence Address:
Dr. P Anandeswari
308/122a, Nesalvar Colony Road, Dadagapatty Gate, Salem - 636 006, Tamil Nadu
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijrc.ijrc_33_22

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Background: Spontaneous sputum smear gave negative results in clinically and radiologically suspected pulmonary tuberculosis (TB) patients. Hence, the study aimed to evaluate the diagnostic yield of induced sputum with direct smear, concentrated smear, liquid culture, and GeneXpert. Materials and Methods: This cross-sectional study was conducted on clinically and radiographically suspected TB patients who were sputum smear-negative; 88 sputum smear-negative patients underwent sputum induction, and the samples were tested for direct smear, concentrated smear, liquid culture, and GeneXpert. Descriptive analysis was carried out for frequency and proportion for categorical variables and mean and standard deviation for quantitative variables. Diagnostic yield was calculated for each test. Data analyzed using coGuide software, V.1.03. Results: The mean age of the study population was 46.87 ± 15.09 years, and male participants were the majority. The diagnostic yield of induced sputum was 19.29% compared to a direct smear. Diagnostic yield was 59.25%, 80.70%, and 94.73% for concentrated smears, liquid culture, and GeneXpert, respectively. Conclusion: Induced sputum gave a high diagnostic yield in smear-negative pulmonary TB. With induced sputum, GeneXpert gave maximum yield compared to direct smear, concentrated smear, and liquid culture.

Keywords: Acid-fast bacilli, GeneXpert, induced sputum, tuberculosis

How to cite this article:
Sivaprakasam A, Gangadharan V, Karthiga G, Anandeswari P. Diagnostic yield of direct smear, concentrated smear, liquid culture, and GeneXpert by sputum induction in smear-negative pulmonary tuberculosis patients. Indian J Respir Care 2022;11:310-3

How to cite this URL:
Sivaprakasam A, Gangadharan V, Karthiga G, Anandeswari P. Diagnostic yield of direct smear, concentrated smear, liquid culture, and GeneXpert by sputum induction in smear-negative pulmonary tuberculosis patients. Indian J Respir Care [serial online] 2022 [cited 2022 Nov 26];11:310-3. Available from: http://www.ijrc.in/text.asp?2022/11/4/310/361030

  Introduction Top

Mycobacterial culture is considered the gold standard for diagnosing tuberculosis (TB), but the final result is usually obtained in 2–6 weeks and requires technical expertise and proper infrastructure.[1],[2],[3] Acid-fast bacilli (AFB) staining by fluorescent microscopy, an essential diagnostic tool, was negative in up to 50% of active pulmonary TB (PTB).[4] Sputum induction is a noninvasive and safe method with a high diagnostic yield for PTB.[5] Hence, the aim was to evaluate the diagnostic yield of direct smear, concentrated smear, liquid culture, and GeneXpert by sputum induction in smear-negative PTB patients.

  Materials and Methods Top

Study population and study site

The study population was patients who attended the Department of Pulmonology at Government Medical College.

Study duration

The study was conducted for 12 months from May 2018 to April 2019.

Inclusion criteria

  • Patients aged 18 years and above have a persistent cough for more than 2 weeks
  • Sputum smear-negative for AFB on two samples and with chest radiographic findings favorable with the diagnosis of active PTB.

Exclusion criteria

  • Active hemoptysis
  • Recent eye surgery
  • Unstable angina or arrhythmia
  • Presence of pleural diseases
  • Those already on antitubercular treatment
  • Uncontrolled asthma or chronic obstructive pulmonary disease.

Study design

This was a cross-sectional study.

Sample size and sampling method

All the 100 patients who were eligible after considering the inclusion and exclusion criteria and attended during the study were recruited by universal sampling.

Ethical considerations

Ethics committee approved the study, and participants gave informed consent.

Data collection tools and clinical examination

Clinical examination

Age, gender, systolic blood pressure (SBP), diastolic blood pressure (DBP), and temperature were measured. Two pulmonologists clinically examined the patients, and radiographs were analyzed during the study. The patients suspected of having TB clinically and radiographically were undergone spontaneous sputum AFB twice. Out of 100 suspected TB patients, 88 were sputum negative, and 12 did not give any result. Hence, the induced sputum samples of these 88 patients were collected and underwent further investigation.

Induced sputum collection

Sputum induction is proposed to improve sample collection and is relatively easy to perform and generally well tolerated.[6] Eighty-eight suspected TB cases at the hospital were isolated in a well-ventilated room. A sputum collection cup was given to the patient on arrival at the hospital. Through an ultrasonic nebulizer (Beurer Ultrasonic Nebulizer IH 50), sputum induction was done using 10 ml of 3% hypertonic saline. Inhalation continued until the patient had produced a good amount of sputum. This induced sputum was used for sputum smear, concentrated smear, liquid culture, and GeneXpert.

Statistical methods

Direct smear, induced sputum smear, concentrated smears, liquid culture, the primary outcome variable were GeneXpert. Demographic variables were considered primary explanatory variables. The diagnostic yield was calculated. Descriptive analysis was carried out by mean and standard deviation for quantitative variables, frequency, and proportion for categorical variables. Data analyzed by coGuide software, V.1.03 BDSS Corp, Bengaluru, Karnataka, India.[7]

  Results Top

A total of 100 participants were included in the final analysis.

The mean age was 46.87 ± 15.09 in the study population. The majority of 64% of participants were male. Of 100 participants, 24 (24%) had diabetes mellitus. The mean SBP, DBP, temperature, and Mantoux were 117.50 ± 17.07 (mmHg), 77.46 ± 10.28 (mmHg), 99.90 ± 0.67 (°F), and 10.78 ± 7.23 min, respectively [Table 1].
Table 1: Summary of baseline parameters (n=100)

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Among the study population, the baseline was a direct smear. After two direct AFB smears, out of 100 samples, 88% were negative, and 12% could not give a result. After inducing sputum, an AFB smear was performed; 11% was found to be positive, and 5% did not provide any result. Induced sputum concentrated smears gave 32% of positive cases, liquid culture gave 46%, and GeneXpert's gave 54% of positive results [Table 2].
Table 2: Summary of outcome parameters (n=100)

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The induced sputum gave a diagnostic yield of 19.29% compared to a direct smear. The diagnostic yield of concentrated smears was 59.25%, liquid culture 80.70%, and GeneXpert 94.73% [Table 3].
Table 3: Diagnostic yield among the induced sputum techniques

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  Discussion Top

The diagnostic yield of induced sputum was 19.29%. GeneXpert had the highest diagnostic yield (94.73%), followed by liquid culture (80.70%) and concentrated smears (59.25%) with induced sputum. In contrast, a meta-analysis has shown that the diagnostic yield of sputum induction for the diagnosis of PTB in individual studies may range from 35% to 95%.[8] A study by Schoch et al. reported that the diagnostic yield for induced sputum on the spot was 46% for smears and 36% for culture.[9] The diagnostic yield for induced sputum was the same in retreatment and new cases in a study by Luhadia et al.; hence, they assumed that the induced sputum might help treat TB cases with scanty sputum.[10]

Sumalani et al. reported that sensitivity was highest for Xpert assay (34.3%), followed by AFB culture (21.6%) and Induced Sputum (IS) AFB smear (7.8%). This result showed that IS Xperts had the highest diagnostic yield for PTB among these investigations.[11] Macías et al. also showed a higher diagnostic accuracy for sputum Xpert assay (65%), followed by sputum culture (43.2%) and sputum smear (26.5%).[12]

Many authors reported a sensitivity of 45% to 83% for sputum smear microscopy and high specificity of 98% or above in clinical practice.[13],[14],[15],[16] Sputum microscopy is considered the most appropriate and particular method to diagnose PTB, and culture-positive status was observed in 22%‒61% of cases with smear-negative results.[17] Lack of quality of sputum sample, low bacterial load (<10,000 bacilli/ml), improper preparation examination of smears, patients with late-stage HIV disease, immune-suppressed patients, and children are the causes of smear negativity.[18] The mortality rate was reported to be 14.1% for smear-negative culture-positive cases. Thus, the early diagnosis of the active smear-negative disease is also essential.[19] When spontaneous sputum is not available, sputum induction or flexible fiberoptic bronchoscopy may be used for diagnosis.[20] In the present study, sputum induction was done.

Induced sputum smear positivity was 11% in this study. In studies by Gopathi et al. and Saglam et al., a positivity rate of 63.3% and 47%, respectively, were reported, which is higher than our study.[20],[21] The average age of the patients was 46.87 ± 15.09 years in this study, and the majority were male (64%). In a study by Schoch et al., the mean age was 38 ± 12.5 years, and the majority were males.[9] In contrast, in a study by Saglam et al., the average age of the patients was 35.8 ± 17 years, and the male-to-female ratio was almost equal.[20] One of the salient features of this study was it included various diagnostic techniques with induced sputum to compare the diagnostic yield.


The small sample size was used due to limited resources and practical constraints.

  Conclusion Top

Induced sputum gave a high diagnostic yield with smear-negative PTB. Hence, the sputum induction technique may be included in the Revised National Tuberculosis Control Programme diagnostic workup of PTB as a routine to improve the diagnostic yield of TB in smear-negative PTB cases to facilitate the initiation of treatment in preventing drug-resistance emergence.


We would like to acknowledge the technical support in data entry, analysis, and manuscript editing by “Evidencian Research Associates.”

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

WHO. Global Tuberculosis Report 2014. World Health Organization; 2014. Available from: http://apps.who.int/iris/bitstream/10665/91355/1/9789241564656_eng.pdf. [Last accessed on 2021 Mar 10].  Back to cited text no. 1
Evans CA. GeneXpert – A game-changer for tuberculosis control? PLoS Med 2011;8:e1001064.  Back to cited text no. 2
Centers for Disease Control and Prevention (CDC). Updated guidelines for the use of nucleic acid amplification tests in the diagnosis of tuberculosis. MMWR Morb Mortal Wkly Rep 2009;58:7-10.  Back to cited text no. 3
Diagnostic Standards and Classification of Tuberculosis in Adults and Children. This official statement of the American Thoracic Society and the Centers for Disease Control and Prevention was adopted by the ATS Board of Directors, July 1999. This statement was endorsed by the Council of the Infectious Disease Society of America, September 1999. Am J Respir Crit Care Med 2000;161:1376-95.  Back to cited text no. 4
Conde MB, Soares SL, Mello FC, Rezende VM, Almeida LL, Reingold AL, et al. Comparison of sputum induction with fiberoptic bronchoscopy in the diagnosis of tuberculosis: Experience at an acquired immune deficiency syndrome reference center in Rio de Janeiro, Brazil. Am J Respir Crit Care Med 2000;162:2238-40.  Back to cited text no. 5
Hepple P, Ford N, McNerney R. Microscopy compared to culture for the diagnosis of tuberculosis in induced sputum samples: A systematic review. Int J Tuberc Lung Dis 2012;16:579-88.  Back to cited text no. 6
BDSS Corp. Released 2020. coGuide Statistics software, Version 1.0, India: BDSS corp. Available from: https://www.coguide.in. [Last accessed on 2021 Dec 22].  Back to cited text no. 7
Gonzalez-Angulo Y, Wiysonge CS, Geldenhuys H, Hanekom W, Mahomed H, Hussey G, et al. Sputum induction for the diagnosis of pulmonary tuberculosis: A systematic review and meta-analysis. Eur J Clin Microbiol Infect Dis 2012;31:1619-30.  Back to cited text no. 8
Schoch OD, Rieder P, Tueller C, Altpeter E, Zellweger JP, Rieder HL, et al. Diagnostic yield of sputum, induced sputum, and bronchoscopy after radiologic tuberculosis screening. Am J Respir Crit Care Med 2007;175:80-6.  Back to cited text no. 9
Luhadia A, Kapur M, Luhadia SK, Sharma RK, Chhabra G, Sharma S. Comparison of induced sputum and fibre-optic bronchoscopy (Fob) in the early diagnosis of sputum smear negative suspected cases of pulmonary tuberculosis under Rntcp settings - A Study conducted in Southern part of Rajasthan. J Pulm Respir Med. 2017;7:2.  Back to cited text no. 10
Sumalani KK, Akhter N, Chawla D, Rizvi NA. Diagnostic yield of sputum induction in patients with pleural tuberculosis at a tertiary care hospital in Karachi. Int J Tuberc Lung Dis 2019;23:1213-6.  Back to cited text no. 11
Macías A, Sánchez-Montalvá A, Salvador F, Villar A, Tórtola T, Saborit N, et al. Epidemiology and diagnosis of pleural tuberculosis in a low incidence country with high rate of immigrant population: A retrospective study. Int J Infect Dis 2019;78:34-8.  Back to cited text no. 12
Hopewell PC, Pai M, Maher D, Uplekar M, Raviglione MC. International standards for tuberculosis care. Lancet Infect Dis 2006;6:57.  Back to cited text no. 13
Gordin F, Slutkin G. The validity of acid-fast smears in the diagnosis of pulmonary tuberculosis. Arch Pathol Lab Med 1990;114:1025-7.  Back to cited text no. 14
Lipsky BA, Gates J, Tenover FC, Plorde JJ. Factors affecting the clinical value of microscopy for acid-fast bacilli. Rev Infect Dis 1984;6:214-22.  Back to cited text no. 15
Woods GL, Pentony E, Boxley MJ Gatson GA. Concentration of sputum by cytocentrifugation for preparation of smears for detection of acid-fast bacilli does not increase sensitivity of the fluorochrome stain. J Clin Microbiol 1995;33:1915-6.  Back to cited text no. 16
A study of the characteristics and course of sputum smear-negative pulmonary tuberculosis. Tubercle 1981;62:155-67.  Back to cited text no. 17
Toman K. What are the main causes of false-positive and false-negative sputum smears? Toman's Tuberculosis. Case Detection, Treatment and Monitoring. 2nd ed. Publisher: World Health Organization, Geneva; 2004. p. 23-7.  Back to cited text no. 18
Narain R, Nair SS, Naganna K, Chandrasekhar P, Rao GR, Lal P. Problems in defining a “case” of pulmonary tuberculosis in prevalence surveys. Bull World Health Organ 1968;39:701-29.  Back to cited text no. 19
Saglam L, Akgun M, Aktas E. Usefulness of induced sputum and fibreoptic bronchoscopy specimens in the diagnosis of pulmonary tuberculosis. J Int Med Res 2005;33:260-5.  Back to cited text no. 20
Gopathi NR, Mandava V, Namballa UR, Makala S. A comparative study of induced sputum and bronchial washings in diagnosing sputum smear negative pulmonary tuberculosis. J Clin Diagn Res 2016;10:C07-10.  Back to cited text no. 21


  [Table 1], [Table 2], [Table 3]


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